1. Field of the Invention
The present invention relates to a laminated balun and, more particularly, to a laminated balun including a matching circuit for making an impedance matching between the balun and a circuit or component connected to the balun.
2. Description of the Related Art
A passive component used in a microwave band of several hundreds of MHz to several GHz, such as a filter, is generally configured to treat a signal in an unbalanced mode. On the other hand, an active component such as an amplifier generally operates in a balanced mode. Therefore, in order to connect the passive component and active component to each other, a balance-unbalance transformer (balun) needs to be used to transform an unbalanced signal into a balanced signal or vice versa. As described above, the balun plays an essential role in a communication device. In recent years, with demand for miniaturization, a small-sized laminated balun with less loss is widely used. Similarly, with regard to a filter which plays an essential role in a communication device, a small-sized one with less loss is widely used.
In the case where the passive component operating in an unbalanced mode, such as a filter, is connected to an external circuit such as an amplifier via a balun, a matching circuit is generally interposed between the passive component and external circuit in terms of loss reduction. Conventionally, in the case where a laminated balun is used, a chip-like matching circuit component including an inductor and/or capacitor is prepared as a different component from the balun and is mounted on a substrate together with an external circuit component such as an amplifier IC. However, the configuration as described above increases the area occupied by mounted components, making it difficult to achieve miniaturization of a communication device. Further, the number of components is increased to increase production cost.
In light of the above, as disclosed in, e.g., JP-2004-320561-A (Patent Document 1), there is proposed a structure in which an impedance matching circuit section is incorporated in the laminated balun. Similarly, JP-2004-304615-A (Patent Document 2) proposes a structure in which a matching circuit is incorporated in the laminated balun.
However, in the method disclosed in Patent Document 1, a dielectric layer formed for the purpose of forming a conducting layer constituting the matching circuit is used, so that the number of dielectric layers to be used is increased to increase the thickness of the balun, which is not desirable in terms of miniaturization. Further, in the method disclosed in Patent Document 2, two matching circuits are formed in the same layer so as to make their characteristics identical, so that the circuit configuration is increased in size. Further, a dedicated dielectric layer for forming a conductive layer constituting the matching circuits is additionally provided to increase the thickness of the balun, which is not desirable in terms of miniaturization.